Environment

Node version: v18.10.0
npm version: v8.19.2
Local ESLint version: v8.25.0 (Currently used)
Global ESLint version: Not found
Operating System: darwin 21.6.0

What parser are you using?

Default (Espree)

What did you do?

Using FlatESLint, the method getRulesMetaForResults thows an error for certain values of the option cwd when passed a result of an anonymous file. This happens whenever cwd is set to a subdirectory of the current directory.

The following repro shows the problem.

import { join } from 'path';

const { default: { FlatESLint } } = await import('eslint/use-at-your-own-risk');

const cwd = join(process.cwd(), 'test'); // subdir 'test' doesn't need to exist for this repro.
const eslint = new FlatESLint({
    cwd,
    overrideConfig: { rules: { 'no-undef': 'warn' } },
    overrideConfigFile: true,
});
const results = await eslint.lintText('foo()');
eslint.getRulesMetaForResults(results); // throws TypeError

What did you expect to happen?

Calling getRulesMetaForResults with the result of an anonymous file should not throw an error, regardless of the engine's cwd.

What actually happened?

The above repro fails with an error like this:

.../node_modules/eslint/lib/config/flat-config-helpers.js:54
    const plugin = config.plugins && config.plugins[pluginName];
                          ^

TypeError: Cannot read properties of undefined (reading 'plugins')
    at getRuleFromConfig (.../node_modules/eslint/lib/config/flat-config-helpers.js:54:27)
    at FlatESLint.getRulesMetaForResults (.../node_modules/eslint/lib/eslint/flat-eslint.js:703:30)
    at file://.../example.mjs:12:8

Participation

• I am willing to submit a pull request for this issue.

Additional comments

What strikes me in the implementation are the different values used to call configs.getConfig in the case of an anonymous file: lintText calls configs.getConfig with a full path like ${cwd}/__placeholder__.js

whereas getRulesMetaForResults ends up calling configs.getConfig("__placeholder__.js")

Then it finds no config and it fails.

Another issue related to this is the unhelpful message of the TypeError.
When getRulesMetaForResults finds no matching config, it could simply tell the user that the results were not created from the current instance of ESLint (not the case here, but still).
The error message is already there, but it is currently only used in a more limited edge case: